An update on the nutritional biochemistry of Selenium and recent developments in Se bioavailability

7/26/2019 An update on the nutritional biochemistry of Selenium and recent developments in Se bioavailability

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S

elenium exists in four oxidation states:

elemental Se (Se0), selenide (Se2),

selenite (Se+4), and selenate (Se+6) in a

variety of inorganic and organic matrices.

The natural inorganic forms, selenite and

selenate, account for the majority of total

global selenium.Organically bound selenide compounds

are predominantly seleno-amino acids; the

principle chemical form of Se in animal tissues is selenocysteine,

while selenomethionine predominates in plants.

The chemistry of selenium resembles that of sulphur in several

respects but these elements are not completely interchangeable in

animal systems.

Both, sulphur and Se occur in proteins as constituents of amino

acids. Sulphur is one of the most prevalent elements in the body

and is present in the sulphur-containing amino acids: methionine,

cysteine, homocysteine and taurine. Selenium is a trace element

and a component of the amino acids selenocysteine andselenomethionine. Selenocysteine is considered the 21st amino

acid in terms of ribosome-mediated protein synthesis.

Selenocysteine is identical to cysteine except that sulphur is replaced

by a Se atom, which is typically ionized at physiological pH.

The presence of selenocysteine in the catalytic site of

Se-dependent antioxidant enzymes enhances their kinetic

properties and broadens the catalytic activity of the enzymes

against biological oxidants when compared with sulphur-

containing species. Selenocysteine (from animal tissues) and

selenomethionine (from plants) are both sources of selenium for

synthesis of SePs.

Replacement of selenocysteine by cysteine in a selenoprotein

usually results in a dramatic decrease of enzymatic activity,

conrming that the ionized selenium atom is critical for optimum

protein function.

Biosynthesis pathway

Signicantly, within all cell types there is a specic biosynthesis

pathway that facilitates selenocysteine synthesis and its

subsequent incorporation into SePs Cellular Se concentrations

are therefore tightly regulated. The regulation of selenoprotein

synthesis is central to understanding Se homeostasis and

disorders following the failure of homeostasis.

Cellular Se concentration is a key regulator of its incorporation

into SePs and acts mainly at the post-transcriptional level inresponse to alterations in Se bioavailability. Selenocysteine

biosynthesis represents the main regulatory point for

selenoprotein synthesis and not absorption as occurs with many

nutrients.

The biochemistry of Se is different from most other trace

elements as it is incorporated in proteins (SePs) at their highestlevel of complexity and function. Selenoproteins incorporate

selenium only in the form of selenocysteine and this occurs

during translation in the ribosome using a transfer RNA specic

for selenocysteine.

Seleno-amino acids (selenocysteine or selenocystine and

by W.L. Bryden, D.D. Moore and S. Shini, School of Agriculture andFood Sciences, University of Queensland, Australia

An update on the nutritional biochemistry of

Seleniumand recent developments in Se bioavailability

A brief history of SeleniumSelenium (Se) is an essential trace element for

animals and humans. It was discovered in 1818

and named Selene after the Greek goddess ofthe moon.

Selenium exerts its biological effects as an

integral component of selenoproteins (SePs)

that contain selenocysteine at their active site.

Some 30 SePs, mostly enzymes, have been

identied, including a series of glutathione

peroxidases, thioredoxin reductases and

iodothyronine deiodinases.

The majority play important roles in redox

regulation, detoxication, immunity and

viral suppression. Deciency or low selenium

status leads to marked changes in many

biochemical pathways and a range of

pathologies associated with defects of

selenoprotein function may occur.

Selenium content of soils can vary widely.

In areas where soils are low in bioavailable

Se, deciencies can occur in humans and

animals consuming plant-based foods grown

in those soils.

Selenium deciency have been reported in

many countries including China, Japan, Korea,

and Siberia, Northern Europe, USA, Canada,

New Zealand and Australia. Within each

country there are large regional differences in

soil Se status and in some localities there are

plants that accumulate Se resulting in selenosisor Se toxicity to grazing animals.

Dietary Se supplementation was rst

permitted some 40 years ago.

Since then, there has signicant advances

in our knowledge of Se metabolism and

the important role that Se plays in animal

productivity and health.

During this period, Se has become an

important addition to dietary supplements for

animals.

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selenomethionine) are required for the synthesis of selenium-

containing peptides and proteins.Importantly, selenomethionine (the major dietary organic

form of Se) that is biochemically equivalent to methionine,

is not incorporated into selenoproteins and therefore, is not a

participant in the regulation of selenium homeostasis. There are

no known human or animal functionally active SePs that contain

selenomethionine.

Only proteins that are genetically programmed and perform

essential biological functions are classied as SePs. Some of

these SePs are enzymes such as the six antioxidant glutathione

peroxidases and the three thioredoxin reductases; the three

deiodinases are involved in thyroid function by catalysing the

activation and deactivation of the thyroid hormones.

Some SePs have direct roles in modulating immunity and

reproductive function, while other SePs facilitate tissue

distribution and transfer of Se.

Selenoprotein P, for example, functions as a transporter of

selenium between the liver and other organs. The functional

characterisation of many SePs remains to be delineated.

Absorption, distribution and metabolic rate

An overview of the metabolism of Se is shown in Figure 1.

Absorption of selenium occurs in the small intestine, where

both inorganic and organic forms of Se are readily absorbed.

Selenite is passively absorbed across the gut wall, while

selenate appears to be transported by a sodium-mediated carrier

mechanism shared with sulphur.

Organic forms of Se are actively transported. The absorption

of selenomethionine is via the same carrier transport protein as

methionine, with competition taking place between methionine

and its seleno analog. Selenium is distributed throughout the body

from the liver to the brain, pancreas and kidneys.

The highest Se concentrations are found in the liver and kidneys

but the greatest total concentration occurs in muscle because of

their proportion of body weight. Selenium is transported by two

SePs; selenoprotein P and extracellular glutathione peroxidase

(GSH-Px).

Other transport mechanisms have been postulated but notdelineated. Only insignicant transitory amounts of free

selenomethionine are found in blood. Following protein turnover,

the released Se, can be recycled via enterohepatic circulation or

excreted. Selenium is eliminated primarily in urine and faeces.

The distribution between the two routes varies with the level of

Figure 1: General pathways (A) of selenium absorption, hepatic

synthesis of selenoprotein P and distribution to various organs.Graphical representation (B) of the optimal range of selenium

required to avoid various human clinical conditions (Adapted

from Kumar and Priyadarsini, 2014)

April 2015 | 39

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exposure and time after exposure.

In ruminants, selenite is the primary

compound available for absorption

because the reducing conditions within

the rumen convert the majority of

selenate to selenite.

In the rumen, about a third of selenite

is converted to insoluble forms that

are passed into manure. Of the soluble

selenite that reaches the intestine, some

40% will be absorbed, compared to

about 80 percent of selenomthionine.

As a consequence of these differences,

in cows, the digestibility of Se from

selenite is around 50 percent compared

to about 66 percent for selenium-

yeast. There is no information on the

impact of the gut microbiota on the Se

requirements of monogastric animals.

Inorganic Se is recognised by the

digestive tissues and is absorbed and

converted into SePs.In contrast, organic Se

(selenomethionine) is not recognized

as Se-containing by mammalian cells.

As a consequence, selenomethionine

is absorbed and metabolized relative to

methionine needs.

If selenomethionine is broken down

within the cell, Se is released and recognized by the cell as a

mineral. It is then processed according to the need for Se.

However, if the cell does not break down selenomethionine, it

may be inadvertently incorporated into a wide variety of proteins

that are not genetically programmed to contain selenium.

The functionality of these proteins will be compromised.

As a metabolic safeguard, neither dietary selenocysteine nor

selenomethionine is directly incorporated into selenoproteins.

All dietary forms of selenium must be metabolised and converted

to selenocysteine and selenoproteins under the genetically

controlled mechanism within the cell.

Much of the absorbed organic Se is transferred into the amino

acid pool, where together with the existing intracellular pool, it is

metabolised by different pathways (see Figure 1). From there, it

is enzymatically converted in the liver to selenide, which serves

as the Se source for selenocysteine synthesis.

Defciency and requirementsSelenium acts biochemically in the animal or bird in a

complimentary manner to vitamin E. Both nutrients prevent

peroxidation of unsaturated fatty acids in cell membranes.

Most of the deciency signs of these nutrients can be explained

by their antioxidant properties. The requirement for each is

therefore inuenced by the dietary concentration of the other.

For example, the Se requirement of the chick is inversely

proportional to dietary vitamin E intake. Thus Se has sparing

effect on the requirement for vitamin E and vice versa.

Manifestation of Se deciency can take many forms and

varies between species. Muscular degeneration or white muscle

disease occurs to varying degrees in all species. In birds,pancreatic brosis is an uncomplicated Se deciency, whereas

exudative diathesis (generalised oedema visible under the skin) is

responsive to both Se and vitamin E.

Pigs with hepatosis diatetica (severe necrotic liver lesions)

are responsive to Se supplements, while both Se and vitamin

E are effective in treating mulberry heart disease (a dietetic

microangiopathy). Reproductive disorders, including retained

placenta in dairy cows, and lowered disease resistance are

observed in all Se decient species. Some species, such as rabbits

and horses, seem to be more dependent on vitamin E than Se for

their antioxidant protection.

This may reect species differences in dependence on non-

selenium containing GSH-Px.

Selenium presents a nutritional conundrum because it is both

essential and highly toxic. There are several approaches to

measuring Se status. These include the measurement of changes

in plasma Se concentration, measurement of GSH-Px enzyme

activity, and absorption/retention studies.

The use of stable isotopes of Se have been used in human

studies and to determine endogenous forms of selenium in foods.

All of these biomarkers are useful indicators of Se status but

because of the role of Se in many biochemical pathways, a single

indicator may not be an appropriate index of Se status.

Dietary supplementation

Selenium is routinely added to animal diets to ensure that

requirements are met.

There has been increased interest recently in Se dietary

supplementation to enrich animal products. The production of

selenium-enriched meat, milk and eggs is viewed as an effective

and safe way of improving the selenium status of humans.

There are a range of products available for dietary Se

supplementation (see Table 1).

Selenium is commonly added to diets as sodium selenite.

However, there has been growing interest in dietary addition oforganic Se. Organic sources are assimilated more efciently than

inorganic Se and considered to be less toxic and therefore more

appropriate as a feed supplement.

Yeast has become the most popular vehicle for the addition of

organic Se because of its rapid growth, ease of culture and high

Table 1: Selenium compounds and their uses in animals and humans

Name and content Nature or origin Uses

Sodium Selenate

Sodium Selenite

Selenase 50 mcg/mL

Synthetic Inorganic

For short-term selenium

supplementation;

orally in the diet, or by injections

for both animals and humans

Biosel

50 mcg/drop

Natural Inorganic For long-term selenium

supplementation in humans

Sintomin BIOSEL 2000

Inactive dry yeast containing

high levels of organicselenium

All animals

Selyeast

Selenomethionine: 1000, 2000, 3000Yeast rich in organic selenium For use as animal feed.

Selemax (1000, 2000)

70 % of total selenium in the form of

selenomethionine

Inactive dry yeast containing

organic seleniumAll animal species and categories

SeLECT

L(+) Selenomethionine &

Vitamin E

Organic, pure

selenomethionine,

Oral administration (capsules)

humans

Sel Plex TM

>50% of total selenium in the form of

selenomethionine

Yeast rich in organic selenium All animals

AB Tor-Sel

selenohomolanthionineYeast rich in organic selenium All animals and humans

L-Selenomethionine

100% L-selenomethionine

Naturally occurring

organoselenium compound

made by plants

Predominate form of selenium

supplement in food for humans;

some use in animals

SeMCTM

Methylselenocysteine 98%

Naturally occurring

organoselenium compound

made by plants

Humans


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capacity to accumulate Se. The major product in selenized yeastis selenomethionine.

Selenomethionine was found to be four times more effective

than selenite in preventing the characteristic pancreatic

degeneration caused by selenium deciency in chicks.

Selenium yeast (selenomethionine) was found to be much more

effective than inorganic Se in increasing the Se concentration

of cows milk. This is in accord with many animal studies and

human clinical trials that have demonstrated the superior efcacy

of L-selenomethionine, in increasing Se muscle content compared

to inorganic Se.

Selenohomoalanthionine (SeHLan; 2 hydroxy-4-

methylselenobutanoic acid) was recently identied in Japanese

pungent radish and has generated much interest as it was less

toxic in human cell culture than selenomethionine. As shown

in Figure 2, differences in metabolism between SeHLan and

selenomethionine may, in-part, explain the apparent difference in

toxicity.

Selenomethionie mimics methionine by sharing the same

metabolic pathways and can replace methionine in peptide

synthesis, as noted above, and thus disrupt protein synthesis.

As shown in Figure 2, the proposed metabolic pathway for

SeHLan appears to be much less complex; SeHLan is only

utilised in the trans-selenation pathway for selenoprotein

synthesis and therefore is not expected to interfere with the

methionine metabolic pathways. The tissue distribution of these

two selenoamino acids may also contribute to differences in

toxicity.

Both are distributed throughout the body with higher liver

and pancreas accumulation of selenomethionine in contrast

to SeHLan which preferentially accumulates in the liver and

kidneys.

At higher doses, selenomethionine has been shown to induce

pancreas damage whereas SeHLan is excreted by the kidneys

without inducing pancreatic damage.

Selenomethionine enriched yeast has been available

commercially for many years.

Recently, a yeast product enriched with SeHLan has becomeavailable and a number of efcacy studies with growing pigs

and broiler chickens have been conducted in Australia with these

selenoamino acid sources.

In the studies both selenomethionine (Sel Plex) and SeHLan

(AB Tor-Sel) were compared to sodium selenite. In the clean

experimental conditions, as demonstrated on many

occasions, dietary supplementation with both the

inorganic and organic selenium resulted in similar

animal and bird performance.

However, tissue accumulation was signicantly

greater when the organic forms of Se were fed,

which is in accord with the literature. Interestingly,

the yeast enriched with SeHLan generated

signicantly higher Se concentrations in muscle

tissue than the selenomethionine enriched product.

The implication of this nding in both pigs and

broilers may imply a greater efcacy of SeHLan in

stressful commercial environments.

Remarks

Seleniums nutritional essentiality was discovered

in the 1950s.

It is now clear that the importance of having

adequate amounts of Se in the diet is primarily due

to the fact that this micronutrient is required for the

biosynthesis of selenocysteine as a part of functional

selenoproteins.Although animals, and presumably humans, are able to

efciently utilise nutritionally adequate levels of Se in both

organic and inorganic forms for selenoprotein synthesis, it is clear

that the bioavailability of Se varies, depending on the source and

chemical form of the Se supplement.

Tissue enrichment with Se is greater when organic forms of the

micronutrient are fed.

Organic selenium, in the form of yeast enriched with

selenomethionine, is widely used in animal nutrition. Recently,

yeast enriched with SeHLan became commercially available

and initial research suggests that it may be more efcacious than

selenomethionine for tissue accumulation of Se.

This has obvious implications for the production of Se enriched

animal products but may also be important in commercial

production units. Greater tissue reserves of Se may enhance an

animals resilience to stress and disease challenge.

Further readingBellinger FP, Raman AV, Reeves MA, Berry MJ. 2009. Regulation

and function of selenoproteins in human disease. Biochemical

Journal, 422:11-22.

.Brennan,KM, Crowdus, CA, Cantor, AH. et al 2011 Effects of

organic and inorganic dietary selenium supplementation on gene

expression proles in oviduct tissue from broiler-breeder hens

Animal Reproduction Science 125: 180 188

Celi P, Selle PH, Cowieson AJ. 2014. Effects of organic selenium

supplementation on growth performance, nutrient utilisation,

oxidative stress and selenium tissue concentrations in broiler

chickens. Animal Production Science 54, 966971.

Fairweather-Tait SJ, Collings R. Hurst, R. 2010. Selenium

bioavailability: current knowledge and future research

requirements. American Journal of Clinical Nutrition,

91:1484S-1491S.

Kumar BS and Priyadarsini KI. 2014 Selenium nutrition: How

important is it? Biomedicine & Preventive Nutrition 4: 333341

Schrauzer GN, Surai PF. 2009. Selenium in human and animal

nutrition: resolved and unresolved issues. Critical Reviews inBiotechnology. 29:2-9.

Tsuji Y, Mikami T, Anan Y, Ogra Y. 2010. Comparison of

selenohomolanthionine and selenomethionine in terms of

selenium distribution and toxicity in rats by bolus administration.

Metallomics. 2:412-418.

Figure 2. Proposed metabolic pathways for SeHLan and

SeMet in animal cells (Source: Tsuji et al. 2010)


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An update on the nutritional biochemistry of Selenium and recent developments in Se bioavailability

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